Many biological assays are carried out with liquids being introduced or retrieved by hand pipetting or by using liquid handling robots that inject liquids into or retrieve liquids from numerous assay devices. While self-sealing septa designs for use in these systems exist, many of them suffer from being overly complicated or less than fully effective at making and maintaining a seal over numerous uses. Thus, a need exists for a simple self-sealing design that offers a reliable, long-term, multi-use seal.